The American soldier would have seen many horrible things in the Soviet Union, human rights violations, prison camps with slave laborers, and an extreme lack to fulfil even the most basic human needs.
Americans are very practical and efficient, so it is likely he would have perceived the system as extremely wasteful (see below for a book recommendation).
He would also have been shocked by the presence of party and NKVD officials in the military hierarchy, and by the power they were wielding over regular soldiers and officers.
He would have seen soldiers sent to the front without weapons, driven forward by by other soldiers and machine gunned down by the Germans without any chance, corpses piling up till they could be used as shields.
There is a chance he would have ended up in the GULag, the Soviet system of concentration camps himself.
Many (all?) Red Army soldiers who were captured ended up in these camps. It is not very likely that the NKVD would have permitted Americans to mingle with Soviets.
During the great depression before WW2, many Americans were lured to the Soviet Union as workers, and most of them ended up in camps, many were even murdered.
One book that could give you some inspiration and a lot of information on real life conditions at that time is
I can recommend this book highly.
It also gives a very good idea how Americans were recruited, this might give you some inspiration.
Another book I recommend is "An American Engineer in Stalin's Russia", more focused on the life of one engineer who had contacts all the way up to Stalin(which probably saved his life).
Here is an article with further links: